Welding technology, which occupies a pivotal position in the manufacturing industry, has been widely used in energy power, transportation, aerospace, marine engineering, heavy machinery and other industries and has become an indispensable manufacturing technology to modern industry. With the rapid development of the national economy, the application of welded products in various sectors of national economy is becoming wider and wider. Replacing welding for riveting, forging, and casting has made welding the dominant method for joining in materials processing and manufacturing. With increasingly high demand on producing high quality welded-products efficiently, traditional welding methods are facing increasing challenges. Highly efficient welding methods are needed to improve welding productivity and weld quality. In recent years, new welding methods have emerged, including high energy laser beam welding, electron beam welding, laser—arc hybrid welding, solid-state welding (Linear friction welding, friction stir welding, etc.). At present, although the welding workload in our country has reached the level as the world leader, our welding efficiency is far lower than those in developed countries. The low degree of automation and lack of efficient welding methods and their implementations are part to claim. Therefore, research and promotion of efficient welding processes and technologies are urgent to improve the competiveness of our manufacturing industry.
Arc welding is a traditional welding method which converts electrical energy into heat energy through the arc process to melt the wire (electrode) or the work-piece to join metals. Arc welding in essence is a heat transfer, mass transfer, and power transmission process. It is a combination of heat, mass and force transmissions. To produce quality welds, such a complex combination must ensure the stability of the arc under challenging operating conditions. With the increased demand on the welding speed, the welding current must be increased in order to improve the melting rate of the wire, i.e., to improve the welding productivity, while reducing the heat input on the work-piece to avoid high-speed associated weld defects. Unfortunately, traditional arc welding processes possess inherent limitation to achieve the desired free control on or free combination of heat input, deposition rate (melting speed) and arc forces acting on the molten pool. The formation and quality of the welds are inevitably affected by undesirable combinations of heat transfer, mass transfer and force transmission. It is thus difficult to guarantee that the welding process would best meet the requirements from various applications. Therefore, efficient welding methods, which apply an automatic control and adjustment of the welding process energy to improve the efficiency of deposition and welding speed while reducing the heat input has become a trend for the development of modern new welding technologies.
Gas tungsten arc welding (GTAW) is a widely used welding process for metal joining Its arc is established between the tip of the non-consumable tungsten electrode and the work-piece with a shielding gas applied to protect the arc and the weld pool area. GTAW can be used in the welding of a wide variety of metals. It is typically used for root passes on pipes and thin gauge materials. Its arc is very stable and can produce high-quality and spatter-free welds without requiring much post-weld cleaning. A typical GTAW system consists of a power supply, a water cooler, a welding torch, cables, etc. For most its applications, direct current electrode negative (DCEN) polarity is used and approximately 70% of the arc heat is applied into the work-piece. Opposite to the direct current electrode positive (DCEP) polarity, the DCEN polarity produces a relatively narrow and deep weld.
However, after the root pass, significant amount of metal is typically needed to fill the groove. Unfortunately, for GTAW, the efficiency to add filler metal into the groove is relatively low. In particular, GTAW currently relies on two most commonly used approaches to fill the metal from the wire: cold wire GTAW process and hot wire GTAW process. In the cold wire GTAW process, the filler wire is directly added as is. To melt the wire faster, in the hot wire GTAW, the filler wire is pre-heated by a resistive heat while it is being fed into the weld pool. This resistive heat is generated by a separate current (typically an alternating-current (AC)) supplied to the filler wire that flows from the wire directly into the weld pool. The current is properly adjusted so that ideally the temperature of the filler wire can reach its melting point as soon as it enters the weld pool. In comparison with the cold wire GTAW, the hot wire GTAW process is more complicated and has a higher cost with the additional power supply, but it can provide a higher deposition rate.
Despite the increased temperature of the filler wire when it enters into the weld pool, the wire melting is still finished by the heat generated from the weld pool during the hot wire GTAW process. That is, part of the heat used to melt the filler wire is essentially absorbed from the weld pool. To melt the wire faster, the arc would have to establish a larger weld pool. Increasing the melting or deposition rate is thus at the expense of an increased weld pool. The arc energy and deposition rate are thus coupled. This coupling reduces the process controllability to provide desirable arc energy and deposition rate freely to meet the requirements from different applications. In addition, for overhead welding where the maximal mass of the weld pool is restricted this coupling also directly reduces the amount of the filler metal that can be added each pass. The productivity is thus directly reduced because of this coupling or undesirable process controllability.
Gas metal arc welding (GMAW) is another widely used arc welding process that can melt the wire highly effectively using an arc spot. If this process is used to melt the wire to deposit metal into the groove, the melting speed can be sufficient. However, in GMAW, the work-piece and wire has the same current. The effective energy consumed on melting wire and that directly applied on the work-piece as additional heat input, where the anode and cathode voltage respectively, hence, wire is deposited at the expense of additional heat which may not be needed by the work-piece. Part of this additional energy becomes a waste. It increases the distortion and material property degradation. This invention thus devices a method to increase this ratio which is referred to as the energy efficiency for deposition application.